Symfony 3.4 is emitting deprecation notices on PHP >=8.1, like

Deprecated: Return type of Symfony\Component\HttpFoundation\ParameterBag::getIterator() should either be compatible with IteratorAggregate::getIterator(): Traversable, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

Symfony 4.4 has PHP 8.2 support and a minimum version requirement of PHP 7.1.3, which matches the minimum version of the current phpBB 3.3.x branch.